1. Field of the Invention
The present invention relates to a rear underbody structure of a vehicle body, and particularly to a rear underbody structure, which has a rear subframe, of a vehicle body.
2. Description of the Related Art
Generally, input force applied from a road surface to a tire is transmitted via a suspension to a vehicle body. In particular, when a vehicle is running on a rough surface road, input force applied to a tire increases. Therefore, vibration of the body increases, and road noise is generated.
On the other hand, in order to reduce road noise, it is necessary only to cut off input force applied from a road surface. However, in this case, control stability of the body deteriorates. Further, although it is also possible to contemplate providing a sound insulation material in a portion of the body producing a sound or increasing the rigidity of a body input portion of a suspension, in these cases there exist drawbacks such as an increase in weight of the body, an increase in cost, and the like.
A rear underbody structure which improves these drawbacks is disclosed in Japanese Utility Model Application Laid-Open No. 2-108672(1990).
As shown in FIG. 13, there is provided a rear underbody structure 70 in which a portion of a rear floor pan 72 in the vicinity of a connecting portion of a cross member 74 and a lower arm 76 swells upward such that a buffer space 78 is formed between the rear floor pan 72 and the cross member 74. Impact load applied from a rear wheel 80 via the lower arm 76 is absorbed by the buffer space 78, thereby causing vibration of the rear floor pan 72 to be controlled so as to reduce road noise.
However, in the above-described rear underbody structure 70, each region 74A in the vicinity of both end portions of the cross member 74 in a transverse direction of a vehicle body is bent upward, and each end of the cross member 74 is connected to a rear floor side member 82. Accordingly, cross-sectional deformation of both end portions 74B, in the transverse direction of the vehicle body, of the cross member 74, which is caused by an input load, increases. With this cross-sectional deformation vibration of the rear floor side members 82 to which both end portions 74B, in the transverse direction of the vehicle body, of the cross member 74 are respectively connected increases. As a result, there are drawbacks in that the vibration of the rear floor pan 72 cannot be sufficiently controlled and road noise cannot be sufficiently reduced.
Further, a rear underbody structure 90 shown in FIG. 14 is disclosed in Japanese Utility Model Application Laid-Open No. 3-26677(1991). In the rear underbody structure 90, a reinforcing gusset 94 is fixed to the inside of a wheel house inner 92 in the transverse direction of the vehicle body and a lower end of the reinforcing gusset 94 is disposed in and fixed to a rear floor side member 96. A cross member 98 is horizontally disposed in and fixed to a region within the rear floor side member 96, in which the reinforcing gusset 94 is to be disposed. A rear subframe 100 is fixed to a bottom surface of respective regions within the left-side and right-side rear floor side members 96. Accordingly, since input force applied from the rear subframe 100 can be transmitted via the reinforcing gusset 94 to the wheel house inner 92, the mounting rigidity of the rear subframe 100 can be improved. Further, since the regions within the left-side and right-side rear floor side members 96, at which the reinforcing gusset 94 is disposed and fixed to the respective rear floor side members 96, interconnect via the cross member 98 and the rear subframe 100, the rigidity of the body input portion of a suspension can be increased.
However, in the above-described rear underbody structure 90, a mounting point S of the rear subframe 100 is located on a main axis H passing through an axial center P of the rear side member 96, and the mounting point S is offset downwardly with respect to the axial center P of the rear side member by a dimension Lz. Accordingly, among input force Fy, which generates from the mounting point S of the rear subframe 100 and acts inwardly in a horizontal direction of the vehicle body, and input force Fz, which acts downwardly in a vertical direction of the vehicle body, the influence of moment FyLz about the axial center P of the rear side member 96 which is caused by the input force Fy increases. As a result, as shown by an imaginary line in FIG. 14, the cross member 98 bends upward and the floor pan 102 is vibrated to produce road noise. Meanwhile, in order to reduce the vibration, a dynamic damper is conventionally mounted on the rear subframe 100 to reduce input force Fy, resulting in an increase in the weight of the body.